1. Field of the Invention
The disclosure relates to a liquid crystal display device that prevents a diffuser plate from being thermally transformed.
2. Discussion of the Related Art
According to the rapid development in information technology, flat panel display (FPD) devices having thin thickness, light weight, and lower power consumption, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display (ELD) devices and field emission display (FED) devices, have been developed and have replaced cathode ray tubes (CRTs).
Among these devices, liquid crystal display (LCD) devices are most widely used for monitors of notebook computers, monitors of personal computers and televisions due to excellent moving images and high contrast ratio.
By the way, the LCD devices require an additional light source because the LCD devices are not self-luminescent. Therefore, a backlight unit is disposed at a rear side of a liquid crystal (LC) panel and emits light into the LC panel, whereby discernible images can be displayed.
Backlight units are classified into an edge type and a direct type according to the position of a light source with respect to a display panel. In edge-type backlit units, one or a pair of lamps are disposed at one side or each of two sides of a light guide panel of a backlight unit. In direct-type backlight units, a plurality of lamps are disposed directly under a display panel.
FIG. 1 is a cross-sectional view of a liquid crystal display (LCD) device including a direct-type backlight unit according to the related art. In FIG. 1, a related art LCD device includes a liquid crystal panel 10, a backlight unit 20, a guide panel 30, a top cover 40 and a cover bottom 50.
The liquid crystal panel 10 includes first and second substrates 12 and 14. The backlight unit 20 is disposed over a rear surface of the liquid crystal panel 10.
The backlight unit 20 includes a reflection sheet 22, a plurality of lamps 24, a diffuser plate 26, and a plurality of optical sheets 28. The plurality of lamps 24 are disposed over the reflection sheet 22, and the diffuser plate 26 and the plurality of optical sheets 28 are sequentially disposed over the plurality of lamps 24. The plurality of optical sheets 28 include a diffuser sheet and a light-concentrating sheet.
The liquid crystal panel 10 and the backlight unit 20 are modularized with the top cover 40, the guide panel 30 and the cover bottom 50. More particularly, the guide panel 30 of a rectangular frame shape surrounds side surfaces of the liquid crystal panel 10 and the backlight unit 20. The top cover 40 covers edges of a front surface of the liquid crystal panel 10, and the cover bottom 50 covers a rear surface of the backlight unit 50. The top cover 40 and the cover bottom 50 are combined with the guide panel 30 to thereby constitute one-united body.
Recently, the LCD devices have been generally used as portable computer monitors, desktop computer monitors and wall-mounted televisions, and LCD devices having a large size and a thin thickness have been widely researched.
As an example, a trial of reducing a distance A between the lamps 24 and the diffuser plate 26 of the backlight unit 20 has been suggested and developed to provide a thin LCD device.
However, to provide the liquid crystal panel with a high quality plane light source, various optical designs are considered. Among the designs, it is important to appropriately maintain the distance between the lamps 24 and the diffuser plate 26. That is, when the distance A between the lamps 24 and the diffuser plate 26 is considerably short, thermal transformation occurs in the diffuser plate 26 due to the heat from the lamps 24 as shown in FIG. 2.
Table 1 shows shifts measured at five points of the diffuser plate 26 according to ON and OFF of the lamps 24. The larger the shift width of the diffuser plate 26 is, the more the diffuser plate 26 has movement. Accordingly, the thermal transformation of the diffuser plate 26 is confirmed. Here the point p3 is a center of the diffuser plate 26, the points p1 and p5 respectively are left and right edges of the diffuser plate 26, and the points p2 and p3 respectively are between the points p3 and p1 and between the points p3 and p5.
TABLE 1p1p2p3p4p5Max.0.872.683.361.002.11Min.−0.32−0.43−0.43−0.50−0.06shift width1.1.193.113.791.502.17
That is, the diffuser plate 26 has the difference in the shift widths at the points p1, p3 and p5 within a range of at least 1.62 to 2.6, and it is noticed that there occurs the thermal transformation in the diffuser plate 26.
As shown in the graph of FIG. 3, the distance A between the lamps 24 and the diffuser plate 26 is not uniform due to the thermal transformation of the diffuser plate 26. The diffuser plate 26 is too close by the lamps 24 in some areas, and light from the lamps 24 has strong straightness. Accordingly, the shapes of the lamps 24 are shown in a displayed image, and lamp mura defects are caused. Thus, the display quality of the liquid crystal display device is lowered.